The methanation proceeds rapidly to equilibrium in the presence of a catalyst, according to one or both of the reactions below, the energy conversion of which is referred to 1 bar and 0.degree. C. ##STR1##
Simultaneously an equilibrium between carbon monoxide and carbon dioxide will set itself as stated below: ##STR2##
Methanation of gases containing small amounts of carbon oxides has been known for a long time. As an example may be mentioned ammonia plants wherein the ammonia synthesis gas, which mainly contains hydrogen and nitrogen, is subjected to a methanation before the proper ammonia synthesis. By the methanation the carbon oxides, which are poisonous to the ammonia catalyst, are converted into methane.
In recent years the methanation of gases containing larger amounts of carbon oxides has been the subject of great interest. Firstly there is an increasing need of preparing methane-rich gases as substitute for natural gas (Substitute Natural Gas, SNG). Secondly, the methanation process is convenient in connection with the transport of chemically bound energy.
Since the reserves of natural gas are limited and since it involves ever increasing costs to win natural gas, it has been attempted in recent years to improve the methods of preparing substitute gases from cheap carbonaceous raw materials such as heavy fuel oils and naturally occurring coal. A number of processes for the gasification of solid and liquid carbonaceous materials are commercially available to-day. These processes have in common that the carbonaceous material is reacted under elevated pressure and temperature with atmospheric air and/or pure oxygen and/or steam. The composition of the product gas from the gasification plant varies from process to process but it predominantly consists of carbon oxides, hydrogen, steam, lower hydrocarbons, mainly methane, and possibly nitrogen. Additionally, the sulfur contents of the raw material will be converted into hydrogen sulfide and/or carbonyl sulfide. Apart from this there will be formed small amounts of low molecular weight organic compounds, i.a. formic acid and hydrogen cyanide. Before this product gas from the gasification plant can be methanated it will be necessary to subject it to various treatments such as removal or conversion of such sulfur compounds and other undesired compounds. For a review of the various gasification processes reference is made to "Ullmanns Encyklopadie der Technischen Chemie", 4th impression, vol. 14, 1977.
In connection with the utilization of nuclear power a new use of the methanation process has become of immediate importance. As is apparent from reaction equations (1) and (2), the formation of methane from carbon dioxides and hydrogen is connected with heat generation and conversely the reaction with steam, the so-called steam reforming, is dependent on the intake of heat. In accordance with this, the heat generated in a nuclear reactor may be used to form carbon oxide (CO and/or CO.sub.2) and hydrogen from methane. In this way the thermal energy is bound and the gases can be transported through pipelines to the places where one wants to utilize this energy. There a methanation is then carried out and the heat generated can be used for electricity production, house warming and other purposes. For a more detailed elucidation of this tropic reference is made to the paper "Transport von Kernwarme mittels chemisch gebundener Energie" by U. Boltendahl et al., published in gwf-gas/erdgas 117 (1976) H. 12, pp. 517-522.
Methanation may be carried out in a number of reactor types differing in principle. The abovementioned methanation of ammonia synthesis gas containing small amounts of carbon oxides is carried out in adiabatic reactors. Such reactors are characterized in general by their simple construction which renders the filling up of catalyst a simple operation. The control of an adiabatic reactor is likewise comparatively simple since the amount of heat evolved is low because of the low content of carbon oxides.
By the methanation of gases having high contents of carbon oxides the amount of heat generated in accordance with the reaction equations (1) and (2) will be so considerable and the temperature thereby so high that the catalyst in an adiabatic reactor may be destroyed, and possibly even the reactor may be damaged. One way of solving this problem involves the cooling and recycling from the outlet of the reactor of part of the methanated gas. Such a process has been described in UK Pat. No. 1,516,319 and U.S. Pat. No. 4,130,575. It is a drawback of this process that considerable amounts of energy are used for the recycling, whereby the total useful effect of the process is reduced.
Another drawback of methanation in an adiabatic reactor is that the enthalpy change by exothermic reactions in accordance with the principle of Le Chatelier will adjust the chemical equilibrium on a gas composition which is disadvantageous in comparison with that desired since the equilibrium concentration of the desired reaction product (methane) decreases with increasing temperature.
Another type of reactor, used in connection with exothermic processes, is the cooled reactor. This reactor most often is constructed as a bundle of parallel tubes in a pressure shell. The catalyst may either be placed in the tubes with the cooling medium round all of the tubes, or vice versa. As cooling medium a large number of liquids with suitable boiling points may be used. Most often one of the heat transfer media called "Dowtherm.RTM." is used. The advantages in using a cooled reactor for methanation i.a. are that the needful amount of catalyst is smaller and that it is possible because of the lower discharge temperature from the reactor to obtain a greater concentration of the desired reaction product (methane) in the outflowing gas from the reactor. When water is used as cooling medium in a cooled reactor, it is a general disadvantage that the steam produced is saturated and therefore it is usable in steam turbines only after having been superheated.